This invention relates to an epoxy optical sheet excellent in optical properties and heat resistance, and a process for producing the epoxy optical sheets at high mass-productivity.
Conventional optical sheets made of epoxy resins are those obtained by pouring an epoxy resin into a space of a cast mold followed by hardening (JP-A-6-337408, JP-A-7-28043 and JP-A-7-120741; the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d). However, it is difficult by this casting method to obtain a sheet having a thickness of 500 xcexcm or less and being free from any optical strain.
In a method wherein a hardened plate made of an epoxy resin is cut into a sheet, optical strain generates in cutting step when a thickness is 500 xcexcm or less. As a result, it was also difficult to obtain an optical strain-free sheet by this method. Thus, epoxy optical sheets having high thickness accuracy at a thickness of 500 xcexcm or less, small retardation and excellent heat resistance have not yet been provided, in spite of a demand therefor.
Moreover, the above-described method suffers from a disadvantage that the complicated steps such as casting and mold opening are required, and this results in poor production efficiency of epoxy optical sheets. Thus, a method is considered, which continuously produces epoxy optical sheets by, for example, a calender method using rolling rolls, a melt extrusion method with T-die or the like, or a flow casting method of a resin solution onto a support, according to the conventional resin sheet production.
However, the epoxy resin involves use as adhesives. Therefore, when the epoxy resin is molded into a sheet by these methods, the epoxy resin adheres to rolls, dies, supports, etc. As a result, it is impossible to easily separate and recover the molded sheets, resulting in breakage of the sheet in separation thereof. In addition, there arises another problem that the thus obtained sheets tend to have uneven surface and thus fail to achieve the required surface smoothness required in optical sheets.
Accordingly, one object of the present invention is to provide an epoxy optical sheet having a thickness of 500 xcexcm or less, an excellent thickness accuracy, a small retardation and a high heat resistance.
Another object of the present invention is to provide a process for producing the epoxy optical sheet at high efficiency.
The present invention provides an epoxy optical sheet comprising a hardened sheet comprising an epoxy resin, and another resin layer formed on one side thereof, wherein the sheet has a retardation of 5 nm or less, an average thickness of 500 xcexcm or less, a thickness accuracy of xc2x110% or less, a glass transition temperature of 170xc2x0 C. or above and a smooth surface.
The present invention further provides a process for continuously producing an epoxy optical sheet, which comprises successively forming a strippable resin layer on a support having a smooth surface, successively spreading an epoxy resin coating solution comprising an epoxy resin, a hardener, a hardening accelerator and a leveling agent onto the resin layer in the form of a sheet, followed by hardening treatment, thereby forming a hardened sheet adhering to the resin layer; and at the same time, recovering the hardened sheet together with the resin layer from the support.